Aretha doesn't sing like she did in her prime? If someone out there is naive enough to believe that she actually
should, I feel sorry for them. Enough of the fairy tale, already.
No amount of auto-tune will give her back her old voice - that's not what AT does. No more than botox and plastic surgery make anyone look younger - it only makes them look like scared old people living in denial.
What's left of Aretha's voice (and I'd venture to say that that's still more than many pop singers in their prime) is usable, imho.
Listening to her on Letterman, I can imagine it would require some comping. Again, probably less than some younger pop stars.
As a matter of fact, before we'd even get into comping, I'd probably resort to the oldest trick in the book: divide and conquer. Plan ahead, chop up the song in parts, take into account that she has shorter breath than in her 20s. Plan multiple passes.
Give her plenty of time. Divide and conquer.
The lady should be treated like the legend she is and given the all time that she needs. Give her a chance to share some of that soul again - even if you must put a bit more time into it. Because that soul is still there.
Unless all you're trying to do is to make a quick $ using her name, regardless of the performance.